An amiloride-sensitive epithelial sodium channel (ENaC) mediates sodium influx across the apical membrane of taste buds cells in the tongue (Heck, et al, Science (1984) 223: 403-405). ENaC, a member of the ENaC/degenerin superfamily of ion channels involved in sodium transport, is composed of three partially homologous α, β, and γ subunits expressed at both the RNA and protein level in fungiform, foliate, and circumvallate papilla as well as the lingual epithelium in taste tissue (Li, et al, Proc. Natl. Acad. Sci. (1994) 91: 1814-1818; Kretz, et al, J. Histochem. Cytochem. (1999) 47(1): 51-64; Lin, et al, J. Comp. Neurol. (1999) 405: 406-420; Xiao-Jiang, et al, Mol. Pharmacol. (1995)47: 1133-1140).
Complementary DNAs (cDNAs) encoding amiloride-sensitive epithelial sodium channel (ENaC) channel subunits have been isolated from kidney cells and expressed in a mammalian cell line. The channel expressed in this system has been shown to have similar properties to the distal renal sodium channel, i.e., high sodium selectivity, low conductance, and amiloride sensitivity. One form of the naturally occurring ENaC channel is comprised of three subunits of similar structure: alpha (OMIM Entry 600228), beta (OMIM Entry 600760), and gamma (OMIM Entry 600761). Each of the subunits is predicted to contain 2 transmembrane spanning domains, intracellular amino- and carboxy-termini, and a cysteine-rich extracellular domain. The three subunits share 32 to 37% identity in amino acid sequence. Alternatively spliced forms of alpha-ENaC have also been identified, indicating heterogeneity of alpha subunits of amiloride-sensitive sodium channels that may account for the multiple species of proteins observed during purification of the channel (See U.S. Pat. No. 5,693,756, which is herein incorporated by reference).
An inhibitor of ENaC sodium channel function, amiloride, is known to attenuate gustatory responses to sodium chloride in numerous non-mammalian as well as mammalian species, including humans (Halpern, Neuroscience and Behavior Reviews (1998) 23: 5-47 and all references cited within; Liu, et al, Neuron (2003) 39: 133-146; Zhao, et al, Cell (2003) 115: 255-266). In humans, amiloride reportedly reduces the intensity of sodium chloride by 15-20% when used at concentrations that specifically inhibit ENaC function (Halpern, Neurosciences and Behavior Reviews (1998) 23:5-47 and all references cited within; Feldman, et al, J. Neurophysiol. (2003) 90(3): 2060-2064). Therefore, compounds that increase the transport of sodium ions through ENaC channels may function as general salt taste enhancers and augment human salt taste perception as suggested in our previous patent application (PCT WO 02/087306 A2). Further, based on published electrophysiological data and the discovery that a human ENaC is expressed in taste bud cells, a model of salty taste transduction mediated by ENaC has been constructed. As such, the use of ENaC in the identification of substances which stimulate or block salty taste perception has been suggested (See U.S. Pat. No. 5,693,756, supra and PCT WO 02/087306 A2).
Cell-based functional expression systems commonly used for the physiological characterization of ENaC are Xenopus laevis oocytes and cultured mammalian cell lines. The oocyte system has advantages over mammalian cells in that it allows the direct injection of multiple mRNAs, provides high levels of protein expression, and can accommodate the deleterious effects inherent in the over expression of ENaC. However, the drawbacks of this system are that electrophysiological recording in Xenopus oocytes is not amenable to screening large numbers of compounds and the Xeropus oocyte is an amphibian not a mammalian system. Studies of the electrophysiological properties of rodent ENaC in mammalian cell lines (HEK293 and MDCK) stably expressing the channel have been reported in the literature. While these studies used mammalian cell lines, channel function was assayed using tedious electrophysiological techniques. Such approaches do not lend themselves to high throughput screening of compounds. Thus, there remains a need in the art for identification of salt taste enhancers amenable to high throughput screening.
The development of salt taste enhancers has been the focus of numerous prior scientific publications and patents. However, direct modulation of the ENaC sodium channel involved in salt taste perception is a novel and unique approach to enhance human salty taste. Some examples of previously reported salt enhancing compounds and their properties are discussed below.
Some proteolzyed proteins, peptides, amino acids, and amino-acid esters reportedly function as salt enhancers (Tamura, et al, Argic. Biol. Chem. (1989) 53(6): 1625-1633, 1989; U.S. Pat. No. 5,711,985). However, these agents require high concentrations, between 30-60 mM, and must be supplemented with hydrochloride acid to positively modulate salty taste. In addition, the cost and difficulty in synthesizing these compounds are prohibitive for their large-scale commercial use as salt enhancers for the general population.
Choline chloride, an ammonium salt classified by the federal government as a GRAS (generally regarded as safe) compound, has been reported to function as a salt enhancer in humans and rodents. In humans, choline chloride increases the saltiness of dilute salt solutions (less than 50 mM NaCl) by a factor of two and reportedly increases the preference or hedonic ratio of both cooked peas and Campbell's low salt tomato soup (Locke, et al, Physiology & Behavior (1994) 55(6): 1039-1046; U.S. Pat. No. 5,260,091; U.S. Pat. No. 5,260,049). However, similar to peptides and amino acids described above, choline chloride requires significant concentrations (in the mM range) to enhance salty taste.
Derivatives of amiloride, which do not block ENaC function but instead block sodium-proton exchange, as well as chloride channel blockers, such as IAA-94 and anthranilic acid, reportedly increase fluid intake, an indirect measurement of salt consumption, in a rodent model system (U.S. Pat. No. 5,260,091). However, the utility of these agents as human salt enhancers has not been reported.
Cetylpyridiunium chloride (CPC) has been reported to increase amiloride-insensitive nerve responses to salt in rats and to enhance the saltiness of low salt Campbell's tomato soup by 50% in humans when used at low concentrations (high uM range) (DeSimone, et al, J. Neurphysiol. (2001) 86: 2638-2641; U.S. Pat. No. 4,997,672). However, CPC is a detergent and based on its structure likely intercalates into lipid bilayers of cells and thereby non-specifically activates salt taste cells by disrupting lipid homeostasis. Indeed, high concentrations of CPC (low mM range), above the critical micelle concentration, actually inhibit rat nerve responses to numerous salty compounds including sodium chloride, potassium chloride, and ammonium chloride, further substantiating that the reportedly observed CPC effects were likely non-specific.
Trehalose, a disaccharide composed of two glucose molecules, reportedly increases the saltiness of sodium chloride solutions by 1.2 to 2-fold (U.S. Pat. No. 6,159,529). Similar to peptides and choline chloride, high levels (1.5-12%) of this sugar are required to enhance saltiness, suggesting that the observed effects could be non-specific and attributable to taste cell volume changes (cell shrinkage) due to hyperosmolarity. In addition, the specificity of trehalose and other aforementioined salt enhancers to enhance salty taste and not modulate other tastes, including sweet, bitter, sour, and umami, was not addressed.
Alapyridaine, a derivative of the amino acid alanine that is formed as a by-product in heated sugar/amino acid mixtures, reportedly decreases the threshold for detecting sodium chloride 5-fold (Soldo, et al, Chemical Senses (2003) 28: 371-379, 2003; Ottinger, et al, J. Agric Food Chem (2003) 51: 1035-1041, 2003). Alapyridaine, however, reportedly functions as a general taste enhancer and decreases the detection thresholds for salt as well as sweet and umami tastes. In addition, the effect of alapyridaine on salt taste at higher, more physiologically-relevant, salt concentrations was not disclosed. Thus, the effects of alapyridaine may only surface when tasting low salt concentrations near threshold detection levels.
The antibiotic novobiocin also reportedly enhances nerve responses to sodium chloride in rats (Feigin, et al, Am. J. Physiol. (1994) 266: C1165-C1172). However, disadvantageously novobiocin reportedly forms amiloride-insensitive cation-selective ion channels in lipid bilayers suggesting that this agent pokes holes in cell membranes and, perhaps similar to CPC, non-specifically increase taste cell activity. The effect of novobiocin on human salt taste perception has not been reported.
Bretylium tosylate, an anti-fibrillary drug that modulates adrenergic and muscarinic receptors, has been reported to specifically potentiate salt taste in rodents and humans without affecting sweet, sour, or bitter taste (Schiffman, et al, Physiology & Behavior (1986) 36: 1129-1137). However, a significant disadvantage of bretylium tosylate, separate from the relatively high concentrations required to positively modulate salty taste (mM range), is that the compound is a therapeutic used to treat cardiac patients. Consequently this compound would be unsuitable for use in the general population.
Glybenclamide, an inhibitor of members of the ATP-binding cassette (ABC) protein superfamily, including the cystic fibrosis transmembrane conductance regulator and the sulfonylurea receptor, reportedly increases amiloride-sensitive ENaC sodium current by doubling the open probability of individual ENaC channels (Chraibi, et al, The Journal of Pharmacology and Experimental Therapeutics (1999) 290: 341-347, 1999; Schnizler, et al, Biochemica et Biophysica Acta (2003) 1609: 170-176). However, because Glybenclamide modulates ABC protein function, it is probable that Glybenclamide effects are due to indirect modulation of ENaC activity by ABC proteins and not attributable to direct modulation of ENaC channel function. In addition, glybenclamide has not been demonstrated to enhance human salt taste perception nor has glybenclamide been suggested as a salt taste enhancer.
Thus, based on the foregoing, it is evident that improved methods for identifying compounds that specifically modulate ENaC and salty taste are needed as are improved salty taste modulators. Preferably, such methods will comprise high or medium throughput methods and will screen for compounds having a direct effect on human ENaC function.